The present invention relates to a pneumatic tire, more particularly to a low noise tire with an improved carcass structure.
In recent years, there is a great demand for low noise tires. It has been known that tire noise decreases if the tire rigidity is decreased. However, this greatly deteriorates the running performance of the vehicle, such as its steering stability. Thus, in order to improve the steering stability, if additional reinforcing layers are disposed in the tire sidewall portions, the tire weight increases and the tire uniformity tends to be disturbed, and furthermore the ride comfort deteriorates. Thus, the decrease in the tire rigidity is nullified.
It is therefore, an object of the present invention to provide a pneumatic tire, in which the noise is reduced without sacrificing the steering stability, ride comfort, and the like.
According to one aspect of the present invention, a pneumatic tire comprises a tread portion, a pair of axially spaced bead portions with a bead core disposed therein, a pair of sidewall portions, a carcass comprising a carcass ply turned up around the bead cores to form a pair of turnup portions and a main portion disposed therebetween. The main portion extends between the bead portions through the tread portion and sidewall portions. The turnup portions are disposed axially outside of the main portion. The carcass ply is made of organic fiber cords extending continuously from one of the ply edges to the other, and is characterized by the carcass cords having a difference in elongation percentage between the main portion and turnup portions. The difference (Ea-Eb) of the elongation percentage Ea in the main portion, from the elongation percentage Eb in in the turnup portions is in the range of from 0.5% to 2.0%.
The elongation percentage means the elongation percentage specified in the "Testing Method for Chemical Fiber Tire Cords" of Japanese industrial standard JIS-L1017, Section 7.7 (Elongation Percentage in Constant Load) and 7.1.1 (Test in Standard Condition).
Preferably, the carcass consists of only the above-mentioned turned-up carcass ply, and the radially outer ends of the turnup portions thereof are disposed radially outward of the maximum tire section width position.
Therefore, in comparison with the turned-up portions, the main portion has a higher elongation percentage, and the carcass cord tension is decreased. As a result, vibrations which are received by the tread portion when running on rough road surfaces are hindered from spreading. Further, the tire resonance frequency shifts downward from the resonance frequency of the car bodies. Thus, the noise can be effectively reduced.
As the turned-up portions have a lower elongation percentage, the tire maintains its precise shape and dimensions, and the running performance such as steering stability can be maintained.
If the difference in the elongation percentage is less than 0.5%, the noise reducing effect can not be obtained. If the difference is more than 2.0%, the noise reducing effect does not increase any more, and the steering stability is lowered.
Another to another aspect of the present invention, a method of making a pneumatic tire, having a carcass ply of organic fiber cords turned up around bead cores to form a pair of turned-up portions and a main portion disposed therebetween, comprises the steps of building a raw pneumatic tire having the carcass ply and vulcanizing the raw pneumatic tire in a mold, wherein the carcass cords are provided with a difference in elongation percentage between the main portion and the turned-up portions, the difference (Ea-Eb) of the elongation percentage Ea in the main portion, from the elongation percentage Eb in the turned-up portions being in the range of from 0.5% to 2.0%. The difference in elongation percentage is provided, for example, by heating the main portion before vulcanizing the tire.